This invention relates to a method of and apparatus for forming and projecting high precision optical images by using a laser-addressed liquid crystal cell and more particularly to a method of quickly creating an uniform dark background by such an apparatus for a full-color display.
As disclosed in U.S. Pat. No. 3,796,999 issued Mar. 12, 1974 to F. J. Kahn and reviewed more recently by Dewey ("Laser-Addressed Liquid Crystal Displays" published in Optical Engineering/May/June 1984, Vol. 23, No. 3) and Todd ("Projection Display Devices" Society for Information Display, Seminar Lecture Notes, Vol. II, Paper 8.1, May 3, 1985), for example, it has been known to make use of a laser-addressed P-type smectic liquid crystal to project an image either for display or for creating a hard copy. For such applications, the liquid crystal is usually sandwiched between parallel transparent substrates to form what may generally be referred to as a cell, or a liquid crystal cell. The liquid crystal cell can be reflective or transmissive. A transmissive cell may be formed, for example, by using two transparent electrodes to sandwich a liquid crystal layer. A reflective liquid crystal cell may illustratively have a transparent electrode on one side of the liquid crystal layer and a light-reflective light-absorbing layer (which, for convenience, will be hereinafter referred to as the reflector-absorber) on the other side such that a light beam incident on the cell from the side of the transparent electrode is reflected while a focused laser beam incident from the opposite side is absorbed and locally heats the liquid crystal.
Let us consider, for example, a reflective liquid crystal cell of the type described above used for projecting images on a screen. One way to write images in the cell, or to cause images to be projected by a projection apparatus of which this liquid crystal cell is a part, is to initially align the liquid crystal molecules in the cell such that their planes are parallel to the layer surface. When the liquid crystal molecules are thus aligned, beams of light incident thereon are specularly reflected from the light-reflective layer and provide, when imaged by a bright field projection system, a bright background on the projection screen. If laser energy is directed from behind to selected areas on the reflector-absorber, the liquid crystal molecules in the addressed areas are heated to become randomly oriented, but are then quickly cooled into a light scattering texture such as focal conic. This causes the projected light to scatter and these areas appear dark on the projection screen. This mode of writing will be referred to as positive writing.
When the background is very bright and written images are very thin, however, such images may not be easily visible on a bright background. For this and other reasons such as superposition of colored images, it is frequently desirable to write by creating light images against a dark background. This mode of writing, being the opposite of the aforementioned positive mode of writing, will be referred to as negative writing. The initial step in this mode of writing is to give the liquid crystal material a light scattering texture over the entire cell area. Thereafter, laser energy is applied as in the case of positive writing to selected areas of the absorber to heat the liquid crystal material in these areas into an isotropic state. An electric field is simultaneously applied across the liquid crystal layer such that the liquid crystal material which has been heated into the isotropic state, while cooling, becomes more uniformly aligned, i.e., less light scattering, but not those molecules outside the laser selected areas which continue to provide a dark background. As a result, the laser-addressed areas appear bright against the initially created dark background.
One method of initially creating a dark background has been to scan all the absorber surface with its laser beam in what may be referred to as a raster mode of writing. This is a very slow process. Another method has been to pas current through one of the conductive layers which is adjacent to the liquid crystal layer and serves as an electrode. When the electrode heats up from the current, the liquid crystal is heated into isotropic phase. Withdrawal of the current causes the liquid crystal to cool back to the smectic phase and, in the absence of a voltage difference across the layer, a scattering layer is formed which appears dark in a bright projection system. This method, however, produces variations in texture sometimes, especially after the cell has been used many times. Still another method has been to incorporate conductive dopants in the liquid crystal material and to pass current across the material.
In prior art, it was necessary to pattern this conducting electrode into resistive strips or to fabricate the electrode with a spatially varying resistance distribution in order to achieve uniform darkening.